Decoupling Temperature-Volume Effects on Poly[2-Methoxy-5-(2'-Ethylhexyloxy)-1,4-Phenylene-Vinylene] Films at the ß-Relaxation Temperature.

In this paper, we report a simple procedure to decouple effects from temperature and volume on the emission properties of thin films of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV). This procedure consists of applying a positive pressure close to the ß-relaxation temperature, Tß (∼220 K), of MEH-PPV, which controls the molecular movement to retain a disordered state for the polymer chains even after the sample is cooled from room temperature. Such decoupling could be confirmed by calculating the photoluminescence (PL) spectra line shape using a semi-empirical model based on molecular exciton and Franck-Condon transitions, and with electron-vibrational modes coupling being parameterized with the Huang-Rhys factor. We also show that the decoupling between temperature and volume effects does not occur if the molecular movement is restricted either by thermal annealing or by depositing the MEH-PPV film on a rigid substrate. This latter finding may be exploited in designing thermally stable devices.

Temperature effect on the electron-vibrational mode coupling of a fully conjugated polyfluorene derivative.

Photoluminescence (PL) and electroluminescence (EL) spectra were used to probe the thermal relaxation processes of the poly(9,9'-n-dihexyl-2,7-fluorenediiylvinylene-alt-1,4-phenylenevinylene) (LaPPS16) electroluminescent polymer. A theoretical model of molecular excitons and Franck-Condon transitions were used to analyze the line shape of the radiative transitions. It was possible to correlate directly the electron-vibrational mode coupling, i.e., the Huang-Rhys parameter, and the polymer relaxation processes due to the effects of molecular dynamics on the electronic states. The results showed different dependences of the thermal relaxation process on PL and on EL due to the molecular dynamics restraints of LaPPS16. This could explain the efficiency variation in organic light emitting diodes where the external electric field would decrease the degrees of freedom of the polymer and activate specific non-radiative channels. Molecular relaxation temperatures of the LaPPS16 polymer are proposed.